The present invention relates to a method for measuring the alignment status of shafts.
The invention also concerns a measurement apparatus of alignment status.
Alignment status refers to the actual position of the shafts of, for instance, cocentrically mounted rotating machines after assembly and during use. The straightness of alignment is an essential factor particularly for the operation of large and rapidly rotating machinery. Such machines include turbine-generator machines of power plants, propulsion systems for ships with screw propeller drives, and different kinds of pump and blower machinery. Problems akin to the measurement of alignment status in rotating machinery are encountered in the deflection and vibration measurements of piping and evaporator equipment and in the deflection measurements performed during the loading and pressurization tests of different machines and pressure vessels.
In most cases the problem of missing measurements of alignment status has been experienced during the vibration measurements of the turbine machinery. The same problem is also encountered with pumps and blowers, because these machines are installed and aligned while being at ambient temperature. In general, a sufficiently widely applicable measurement method capable of taking into account deflections caused by the heating of machinery has been lacking. Practical experience has, however, revealed that the relative displacements between the bearings as well as the bearings and their yokes cause substantial vibration problems generally invoked by the incorrect distribution of bearing loads between the individual bearings or by the friction of labyrinth ring seals and other types of seals. If it were possible to measure the changes in the alignment status during the running of the machinery in a reliable manner, the alignment could be adjusted to match the actual operating conditions to a higher degree, thereby allowing for a reduction of vibrations and wear in the machinery.
Measurement of the alignment of a shaft line is conventionally performed using, for instance, the following methods:
A "liquid level" developed by ENEL is based on the conventional principle known from levelling equipment. This equipment is intended for continuous monitoring. The device has a complicated construction which is difficult to install and can't be moved. Further, the equipment is incapable of measuring movements in the horizontal plane. In regard to its operating principle, the system is inaccurate.
A laser-optical meter called "Permalign" is comprised of a laser and a beam detector. The laser is attached to either of the machines or bearing yokes and the detector cell is attached to the other. The device is applicable for continuous monitoring of two adjacently located bearings, while, however, unsuitable for the alignment status measurement of the entire shaft line in the turbine machinery. The equipment can only measure the relative displacements between any two points.
Disclosed in the publication WO 85/05443 is a measurement equipment for the relative displacement of two machines with a cocentrical shaft line. This equipment measures the displacements with the help of three measurement elements, which are comprised of a light source, a reflection prism and a beam detector. The light source and the beam detector are mounted to one of the machines, while the reflection prism is attached to the other machine. The displacements are measured by directing a light beam onto the prism which reflects the beam to a detector placed adjacent to the light source. The displacements of the machines in relation to each other can be computed from the displacement of the reflected beam. This method provides only for the measurement of the relative mutual displacement of two machines.
A measurement method based on the use of a laser is disclosed in U.S. patent publication 3,902,810 featuring a capability of simultaneous alignment of several machines with cocentrical shafts during assembly. This method does not lend itself to the measurement of displacements during the running of the machinery.
Thus, none of the commercially available measurement equipment is universally applicable to the alignment status monitoring of an entire turbomachinery outfit.